Self-piercing female fasteners or pierce nuts were invented and developed by the predecessor in interest of the assignee of this application over fifty years ago as disclosed, for example, in U.S. Pat. No. 2,707,322. A pierce nut is driven by a reciprocable plunger to pierce its own opening in a panel and the pierce nut is then permanently secured in the panel by a die member. The pierce nut disclosed in this patent has been commercially successful, particularly for mass production applications for attachment of one component to a second component by the automotive and appliance industries. In a typical application, the pierce nut is installed in a metal panel by an installation head commonly installed in the upper die shoe or die platen of a die press and the die member or die button is installed in the lower die shoe or die platen of the die press. The panel to which the pierce nut is installed may be simultaneously formed by the die press. Numerous improvements have been made to the pierce nut disclosed in the above-referenced patent, including improved retention by providing dovetail-shaped grooves on opposed sides of the pilot portion as disclosed, for example, in U.S. Pat. No. 3,648,747, also assigned to the predecessor in interest of the assignee of the present application. Finally, in many mass production applications, it is preferable to assemble the pierce nuts in a continuous frangible strip as disclosed, for example, in U.S. Pat. No. 3,845,860, which is also assigned to the predecessor in interest of the assignee of the present application.
The pierce nuts of the type disclosed in the above-referenced U.S. patents are formed by rolling the desired cross-sectional configuration of the pierce nuts, generally starting with a round wire, as known by those skilled in this art. The round wire is rolled, flattening opposed sides of the wire and the wire is then continuously rolled into the desired cross-sectional configuration of the pierce nut, generally starting with a round wire, as known by those skilled in this art. The round wire is rolled by flattening opposed sides of the wire and then rolled into the desired cross-section of the pierce nut, including a rectangular central projecting pilot portion and rectangular flange portions on opposed sides of the pilot portion as a result of the rolling process. In the Hi-Stress® pierce nut available from the assignee of this application, grooves are rolled into the top faces of the flange portions on opposed sides of the pilot portion, preferably having at least one inclined groove side wall, referred to as a “re-entrant” groove, which significantly improves the retention of the pierce nut on the panel following installation. The rectangular pilot portion of the nut strip or nut section is pierced or punched, forming a generally cylindrical bore and the pierce nut section is then cut to length between the bores and the bores may be tapped or internally threaded, if desired. Alternatively, the pierce nut bore may remain unthreaded for receipt of a thread forming or thread rolling male fastener.
As will be understood by those skilled in this art and shown in the above-referenced U.S. patents, the pierce nuts formed by the rolling process described are necessarily rectangular and include a rectangular pilot portion having relatively sharp corners which could create or propagate stress risers and cracks in the metal panel during and following installation. Cracking of the panel may reduce the integrity of the nut and panel joint and the joint may leak making the installation unsuitable for a sealed joint. There has therefore been a long felt need for a pierce nut of the type disclosed in the above-referenced U.S. patents which may be formed by a rolling process, but which eliminates the sharp corners of the pilot portion and thus eliminates or reduces the propagation of stress risers and stress cracking. Various attempts have been made by the Applicant and others to eliminate the sharp corners of the pilot portion as disclosed, for example, in co-pending application Ser. No. 10/691,677 filed Oct. 23, 2003, the disclosure of which is incorporated herein by reference.